Ceramic Coatings for Nuclear Fuel Cladding to Enhance Accident Toleranc

This research is focused on developing nuclear fuel claddings with ceramic coatings that can perform well during normal operation and can withstand transient conditions such as loss of coolant accident for longer durations than current cladding material and then provide longer coping times. This was done by creating TiAlN coatings using physical vapor deposition and optimizing the deposition parameters and multilayer architecture to achieve the best performance.;Zirconium-based alloys are currently widely used as cladding materials due to their low neutron absorption cross-section, good mechanical properties and high melting point. However, waterside corrosion of these alloys cause zirconium oxide formation and hydrogen generation which leads to hydrogen pick up and hydride embrittlement. Moreover, hydrogen generation in case of accelerated corrosion at higher temperatures due to loss of active cooling can lead to hydrogen explosions such as observed in the Fukushima-Daiichi accident when hydrogen explosions in the reactor building worsened the accident conditions. This accident motivated research into Accident Tolerant Fuels (ATF), which are fuels that are more forgiving in case of a loss-of-coolant-accident (LOCA). This research is an innovative approach since it considers application of TiN and TiAlN ceramic coatings on ZIRLORTM substrate by cathodic arc physical vapor deposition (CA-PVD), which improve corrosion resistance without a major change in core design and contribute to the design safety.;Cathodic arc physical vapor deposition was used since it provides flexibility in coating properties by adjustment of deposition parameters. A systematic study was performed to identify the optimum deposition parameters to achieve enhanced adhesion of nitride-based coatings on ZIRLORTM substrates and best corrosion performance. The developed coatings are subjected to scratch tests and long-term corrosion tests. First, the single-layer TiAlN and single-layer TiN coating deposition on ZIRLORTM sheets were characterized in detail with regards to their as-deposited coating properties (topography, uniformity, crystal structure, residual stresses), failure modes during scratch testing and oxide formation after corrosion testing. Second, a multilayer coating design architecture is investigated to achieve enhanced corrosion resistance. Then the 8- layer TiN/TiAlN coatings deposited on ZIRLO RTM tubes were exposed to long-term corrosion testing. Throughout the study, the following parameters were optimized to provide best corrosion resistance: (i) substrate surface roughness, (ii) substrate surface preparation method, (iii) titanium bond coating layer thickness, (iv) total coating thickness, (v) cathode composition, (vi) substrate bias, (vii) nitrogen partial pressure and (viii) multilayer design architecture.;Mechanical performance evaluation involved scratch testing and post-scratched sample failure mode characterization. The corrosion tests were performed at Westinghouse in autoclave in static pure water at 360°C and 18.7 MPa up to 128 days in order to evaluate the normal operating condition performance of the coatings. Furthermore, supercritical water testing was performed in University of Michigan autoclave at in deaerated water at 542°C and 24.5 MPa for 48 hours. Additionally, differential scanning calorimetry and thermogravimetric analysis were performed to test oxidation onset point in air atmosphere. Furthermore, high temperature air oxidation testing was performed in furnace in air atmosphere up to 800°C. Weight gain analysis and characterizations (optical microscopy, X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, Raman spectroscopy) were performed to examine as-deposited coating properties and to evaluate coating performance after corrosion and mechanical testing.;The results determined that 8-layer TiN/TiAlN coatings deposited with optimized parameters achieved good adhesion and substantially enhanced corrosion performance, which makes this approach promising for development of accident tolerant nuclear fuel cladding.